Testing promise rejection with Mocha

promise-test.js

const { assert } = require('chai');

function isError(e) {
    if (typeof e === 'string') {
        return Promise.reject(new Error(e));
    }
    return Promise.resolve(e);
}


describe('Promise Testing', () => {
  // With done callback - 8 lines
  it('should test rejections using done', (done) => {
    aMethodThatRejects()
      .then(() => {
        done(new Error('Expected method to reject.'))
      })
      .catch((err) => {
        assert.isDefined(err);
        done();
      })
      .catch(done);
  });
  
  // With built in Promise support - 8 lines + helper
  it('should test rejections using catch', () => {
    return aMethodThatRejects()
      .then(() => {
        return Promise.reject('Expected method to reject.');
      })
      .catch(isError)
      .then((err) => {
        assert.isDefined(err);
      });
  });
});

it('should test rejections using then', () => {
  return aMethodThatRejects()
    .then(
      () => Promise.reject(new Error('Expected method to reject.')),
      err => assert.instanceOf(err, Error)
    );
});

People often forget that .then(onFulfilled) is actually .then(onFulfilled[, onRejected]).

thanks @naptowncode, this is really the best solution

@iamdanthedev, consider:

chai = require('chai')
  .use(require('chai-as-promised'))

expect = chai.expect
  
it('should test rejections using then', () => {
  return expect(aMethodThatRejects()).to.be.rejectedWith(ErrorClass, "message")
});

there is similar functionality Jests expect and in power-assert

and if you are running on node 10, there is native assert.rejects

Using node's assert module I was able to do this:

    it('Should reject on bad values', async () => {
            const badValues = async () => {
                // A method the rejects when given a bad argument.
                myMethod('bad argument');
            }

            assert.rejects(badValues);
    });

@jfavrod this does not assert on error message.